The present invention relates to technique for controlling combustion changeover in an internal combustion engine among a plurality of combustion modes having different combustible ranges of an equivalent ratio (fuel air ratio/theoretical fuel air ratio), and more specifically to system and method for controlling a combustion changeover timing for an engine equipped with an EGR (Exhaust Gas Recirculation) system.
Recently, the technique of in-cylinder direct fuel injection in a spark ignition engine such as gasoline engine is under development to improve the fuel efficiency and emission performance by selectively using stratified charge combustion and homogeneous charge combustion.
In a low and medium load region, a control system for such a type operates an engine in a stratified charge combustion mode by injecting fuel directly into a combustion chamber during the compression stroke so as to produce a stratified combustible mixture closely around the spark plug. The thus-achieved stratified combustion enables stable combustion with an ultra lean mixture, and hence significant improvement in the fuel efficiency and emission control performance of the engine.
In a high load region over a predetermined engine load, the engine is operated in a homogeneous charge combustion mode to meet a demand for higher output torque. In the homogeneous combustion mode, fuel is injected during the intake stroke so as to produce a homogeneous air fuel mixture. (In some examples, a fuel injection valve is provided separately in an intake port.)
The control system changes over the combustion mode between the stratified combustion mode and the homogeneous combustion in accordance with one or more engine operating conditions.
To reduce NOx emission, an engine for a vehicle is generally equipped with an EGR system for recirculating part of the exhaust gases from the exhaust system to the intake system. A desired target EGR rate is not equal between stratified combustion and homogeneous combustion. Therefore, there is a need for switching the target EGR rate in accordance with a changeover of the combustion. In general, EGR is carried out in the stratified charge combustion mode and homogeneous stoichiometric combustion mode while EGR is cut off in a homogeneous lean combustion mode to maintain stable combustion. An EGR control system of such a type is shown in Japanese Patent Provisional (Kokai) Publication No. 7(1995)-269416.